br Voltage dependent anion channel
Voltage-dependent anion channel VDAC is a ubiquitous protein showing well conserved structural and functional properties in spite of major variations in its sequence (for review see ). Most of what we know about VDAC electrophysiological properties (for reviews, see , ) was initially obtained from mitochondrial proteins using artificial reconstitution in lipid bilayers or liposomes, very seldom under “physiological” conditions, never in blood cells. Their presence in ONC201 membranes was demonstrated more recently , . Maximal conductance levels of 4–5 nanoSiemens or 350–450 pS are obtained with molar concentrations of NaCl or KCl or with more physiological concentrations (0.15M), respectively , . Conductance and selectivity are voltage-dependent and an important point to keep in mind is that, in these artificial systems, the channel protein is stable and remains open at low voltages, say between −10mV and +10mV, whereas at higher positive and negative potentials they display multiple sub-conductance levels of varied permeabilities and selectivities , . At high conductance levels, currents are carried by small ions (Na+, K+, Cl−,…) as well as large anions (glutamate, ATP, …) and large cations (acetylcholine, dopamine, Tris, …) with a global preference for anions in saline solutions composed of NaCl or KCl. At the opposite, at higher voltages, currents are carried exclusively by small ions with a marked preference for cations and specially for Ca2+ in the so-called closed-states corresponding to low conductance levels , , , . VDAC may be found in different oligomerization states from monomers to hexamers. Its activation and regulation mechanisms remain unclear. The “reactive oxygen species”(ROS) and Ca2+ ions were identified as regulators and compounds such as DIDS, La3+, Tb3+, Ruthenium red (interacting with Ca2+ binding sites) increase the frequency of closure episodes . VDAC is phosphorylated by PKA, PKC and GSK3 , . In “resting” conditions the RBC membrane potential is ~−12mV but the anion channel is silent, which is in apparent contradiction with the above description of VDAC's behavior. It simply means that the RBC's VDAC is controlled by specific gating mechanisms which remain to be identified. Although the question cannot be considered finally settled, it has been shown that the presence of human serum contributes to maxi-anion channel activation  and it seems probable that some agents present in serum underlie physiological processes that are important to VDAC gating. In addition, VDAC presents 3 isoforms in mammals : VDAC1, 2, 3. VDAC1 is the most abundant and most studied isoform whereas VDAC3 is predominant in the RBC membrane (Bouyer et al., submitted) and seems to exhibit different kinetics, gating mechanisms and permeability properties, compared to the two other isoforms.
TSPO and ANT VDAC is endowed with binding sites allowing interactions with other constituents of the cytoskeleton and with the other proteins constitutive of the PBR complex TSPO and ANT. The TSPO gene is uniformly distributed in human cells and abundant in mitochondria (for a review see ). The recent discovery of a TSPO2 sub-family overexpressed in hematopoietic cells  suggests a peculiar participation of this protein in RBC maturation. TSPO is involved in steroidogenesis, mitochondrial respiration, heme metabolism, modulation of calcium channels and cellular growth . TSPO binds the three PBR ligands and plays a major role, in association with VDAC, in initiation of the mitochondrial apoptosis process . The mechanism of interaction with VDAC remains hypothetical but could involve ROS which are possible activators of VDAC and are known to be generated and transported by TSPO . ANT is the most abundant protein in mitochondrial inner membrane and plays a dual role as ATP/ADP exchanger and as a key actor with VDAC and TSPO in the apoptotic process . In brief, we know very little about the role played by the PBR complex in RBC physiology, but, according to the known properties of its three components, we can predict a major role in RBC membrane transports, volume, electrolyte, redox and acid–base regulations.